A boxcar is loaded with cases of dog food, rolled onto a platform of complex machinery, and given a thorough shaking. Then it is rolled off and unloaded . . . only to go through the whole process again.

This is all part of the center's mission of "conducting comprehensive testing , evaluation, and associated development of ground transportation systems."

In many ways the history of the Transportation Test Center (TTC) parallels the aspirations and the hard realities of the US rail transportation scene.

The facility, with its current $22 million budget, was opened in 1971 as part of a program to develop and test 300-mile-per-hour transit systems. Among the futuristic machines that have raced around its 55 miles of track are a tracked, air-cushion vehicle and a linear induction-driven car (LIMRV), which set a world speed record for steel wheels on steel rails of 255.4 m.p.h.

In 1974, the same year that the LIMRV set its speed record, TTC's mission was changed. No longer were these futuristic systems considered practical. Instead , the center's work was reoriented to the more immediately practical. It began testing new subway cars for the Urban Mass Transit Administration. It started a joint industry-government program for wear-testing road beds, fasteners, ties, wheels, and other standard, freight-car equipment. This is the train which runs from dusk to dawn.

Today the future of the test center is in doubt. "There are a number of policy changes in the wind that could have a major effect on us," acknowledges its director, Edward Mathews.

A great deal of the effort at TTC in recent years has been devoted to testing AMTRAK locomtive and cars. For instance, the center just finished testing a new , high-speed locomotive for the Northeast Corridor. If current Reagan proposals to get the federal government out of the railroad business bear fruit, the center would loose a considerable amount of work.

On the other hand, Mr. Mathews believes TTC's transit work will remain relatively steady. A number of transit authorities are getting new cars and the center will be testing them all.

The industry doesn't always rush to adopt innovations the center's tests have proved worthwhile. An example, pointed out by TTC researcher Rick Kilpatrick, is steel-banded aluminum wheels on transit vehicles. "They save a lot of weight and therefore energy. In transit applications there weren't any problems. I really don't understand why they haven't been picked up," he says, bemusedly.

The Facility for Accelerated Service Testing has been more successful. Among other things, its work has helped convince many railroad people that concrete ties, although more difficult to install, are worth the extra effort, particularly on curves, and that certain grades of stell rails wear much better than others, says Mathews.